The present invention relates to a power train system for a vehicle, particularly of the kind having a planetary reduction gear unit and a parallel reduction gear unit and adapted to transmit power from a motor such as an electric motor to the parallel reduction gear by way of the planetary reduction gear unit and then from the parallel reduction gear unit to a differential gear unit which is disposed so that an output shaft of the motor and an axis of rotation of an output portion of the differential gear unit are parallely offset in the longitudinal direction of the vehicle.
A prior art power train system for a vehicle is disclosed by way of example in Japanese Patent Provisional Publication No. 10-325456.
It was revealed by the applicants that the above described prior art power train system had two problems. A first problem is that a rotation shaft of a smaller gear of a parallel reduction gear unit requires four bearings in order to support the load perpendicular thereto. In this connection, in the above described prior art power train system, a sun gear of a planetary reduction gear unit has a hollow rotation shaft connected at a closed end to a motor. The rotation shaft of the smaller gear has an end portion supported on a stationary casing by way of a bearing and another end portion protruded into the hollow rotation shaft of the sun gear and supported on the inner circumferential surface of the hollow rotation shaft of the sun gear by way of a pair of bearings. The hollow rotation shaft of the sun gear is in turn supported on the casing by way of a bearing. The necessity of a large number of bearings inevitably results in a high cost of the power train system. Further, a friction loss resulting due to a load perpendicular to the rotation shaft of the smaller gear is caused at the respective four bearings, thus causing a large total friction loss.
A second problem is that since the rotation shaft of the smaller gear is rotatably supported at an end portion on the inner circumferential surface of the hollow rotation shaft of the sun gear, there is a difficulty in making smaller the diameter of the sun gear formed on the outer periphery of the hollow shaft and therefore in making smaller the size of the planetary reduction gear unit.
In this instance, the planetary reduction gear unit is naturally restricted in the outer diameter of a ring gear due to the requirement for its mounting on a vehicle (e.g., layout of a vehicle) and the weight requirement. Due to this, when the diameter of the sun gear is large, it is difficult to make smaller the value a (the number of teeth of the sun gear/the number of teeth of the ring gear), thus making it difficult to attain a large reduction ratio.
Accordingly, since the reduction ratio xe2x80x9crxe2x80x9d is small, the above described prior art power train system requires the motor to produce a large torque, thus requiring such a motor that is large-sized, heavy and expensive.
In brief, the above two problems are caused by the prior art structure wherein an end of the rotation shaft of the smaller gear is supported indirectly upon the casing, i.e., by way of the hollow rotation shaft of the sun gear in addition to a bearing means.
It is accordingly an object of the present invention to provide a novel and improved power train system for a vehicle which can reduce the number of bearings necessary for supporting a load perpendicularly to an axis of rotation of a smaller gear of a parallel reduction gear unit and can make a planetary reduction gear unit compact in size.
To achieve the above object, there is provided according to the present invention a novel and improved power train system for a vehicle having a motor and a differential gear unit. The power train system comprises a stationary casing, a planetary reduction gear unit disposed within the casing and having a sun gear disposed coaxially with and connected to an output shaft of the motor so as to be driven thereby, a ring gear fixedly attached to the casing, at least one pinion gear disposed between the sun gear and the ring gear and a carrier carrying thereon the pinion gear and having an output portion, and a parallel reduction gear unit disposed within the casing and having a smaller gear and larger gear meshing with each other. The smaller gear has a rotation shaft connected coaxially with the output portion of the carrier. The larger gear is disposed coaxially with an output portion of the differential gear unit which is disposed so that the output shaft of the motor and an axis of rotation of the output portion of the differential gear unit are parallely offset in the longitudinal direction of the vehicle. Opposite ends of the rotation shaft of the smaller gear are supported rotatably and directly upon the casing.